Fix some metal material thermochemical properties

[cake-pie]

Summary

Bugfixes "Fix some metal material thermochemical properties"

Purpose of change

Rectify errors in properties of certain metals in materials.json, primarily latent_heat but also specific heat capacity for zinc.

Because numbers may vary slightly depending on the source used or due to rounding after computation with source figures of different numerical precision, small discrepancies (e.g. a couple J/kg) were ignored.

Describe the solution

Check preexisting values by comparison against two sources:

Latent heat of fusion
https://www.engineeringtoolbox.com/fusion-heat-metals-d_1266.html
additional verification computed from data in wikipedia infobox (heat of fusion (kJ/mol) divided by atomic weight)

Specific heat capacity (solid)
https://www.engineeringtoolbox.com/specific-heat-metals-d_152.html
additional verification computed from data in wikipedia infobox (molar heat capacity (J/mol K) divided by atomic weight)

• Iron: "latent_heat": 247, // = 13810/55.845 (was: 273)
• Lead: "latent_heat": 23, // = 4770/207.2 (was: 2)
• Silver: "latent_heat": 105, // = 11280/107.87 (was: 111)
• Platinum: "latent_heat": 114, // = 22170/195.08 (was: 100)
• Zinc: numbers were blindly copied from aluminum in RPG dice #31031.

    "specific_heat_liquid": 1.18, // help wanted: this number is definitely wrong
    "specific_heat_solid": 0.39,  // = 25.47/65.38 (was: 0.91)
    "latent_heat": 112,           // = 7320/65.38 (was: 260)

I don't have a source for specific heat capacity in liquid phase, so I've inserted a comment in the file to flag up the fact that the number is definitely erroneous.

Describe alternatives you've considered

Not really, dealing with factual information here.

Testing

Validate JSON to ensure nothing accidentally broken while editing the file

Additional context

Only elemental metal materials were checked, alloys and other materials were not included.